A typical conventional system for introducing sheets from two different directions to predetermined positions of different heights and dropping the sheets therefrom in one and the same place is disclosed in Japanese Utility Model Application No. 61-43246 (published under No. 62-157753) for an invention entitled "System for Supplying Sheets into a Case". The system disclosed therein includes a conveyor located above a case and having upper and lower sheet holders. The lower sheet holder holds one end of a first sheet provided on a left-hand side support and carries the sheet to a first predetermined position above the case, while the upper sheet holder holds one end of a second sheet provided on a right-hand side support and carries the sheet to a second predetermined position above the first predetermined position. Then, the two sheet holders release the sheets to drop the sheets into the case. Since the first sheet is nearer to the bottom of the case, first the first sheet, and then the second sheet, come into the case. This operation may be reiterated a number of times to stack a number of sheets from the opposed directions in one and the same place.
Since the first sheet is nearer to the bottom of the case, the first sheet may drop relatively exactly on a predetermined position in the case. However, since the second sheet has a longer distance to the case than the first sheet, it is quite possible that the second sheet may not drop straight on the first sheet in the case, but may meet sufficient air resistance to deviate the second sheet from the straight path in a horizontal direction as it drops, with the result that the second sheet only overlaps the first sheet instead of overlying the first sheet exactly. The portion of the second sheet which does not overlie the first sheet must be cut out. It is a waste of the material and results in a smaller yield. If such a portion is not cut out, such a portion may go against parts of a conveyor which conveys the stacked sheets to a subsequent stage, thus creating trouble.